Displays in image display devices have been shifted from conventional cathode-ray tube displays to thin displays, such as liquid crystal displays and plasma displays. With the thin displays, thicknesses of the image displaying devices can be decreased. Liquid crystal panels used in the liquid crystal display devices do not emit light. Therefore, liquid crystal display devices including liquid crystal panels require backlight units. The backlight units are classified broadly into a direct type and an edge-light type based on mechanisms. The edge-light type backlight unit includes alight source, alight source printed circuit board, and a light guide plate. The light source is mounted on the printed circuit board. The light guide plate includes a light entrance surface and a light exit surface. The light entrance surface is opposite the light source and through which light enters the light guide plate and the light exits the light guide plate through the light exit surface. Patent Document 1 discloses an example of a liquid crystal panel display device that includes such a backlight unit.